The answer to the riddle is based on Diogenes Laërtius’ account of Thales’ answers to two questions:

When Thales was asked what was difficult, he said, “To know one’s self.” And what was easy, “To advise another.”

The anachronistic picture of Thales is from the Nuremberg Chronicle, a history of the world published in 1493 under the title Liber Chronicarum. I guess any other image, no matter how authentic it might appear to us, would be almost as anachronistic.